Diverter systems for use in subsea drilling into hydrocarbon wells are well known. Originally, diverter systems were installed on drill ships or semi-submersible drilling rigs in order to handle shallow gas when drilling with a marine riser on top hole sections before the Blow-Out Preventer (BOP) was installed. Today it is more common to drill the top hole sections with seawater or water based mud and with return to seabed or “riserless” return to the rig.
Today, the main purpose of the diverter system is to handle gas that for some reason has entered the riser after the BOP is shut in on a so-called “kick”. A kick is a situation where hydrocarbons, water, or other formation fluid enters the wellbore during drilling, because the pressure exerted by the column of drilling fluid is not great enough to overcome the pressure exerted by the fluids in the formation being drilled. As the industry is going to deeper water it has been more difficult for the drillers to detect a kick early because the gas will be in liquid or dense phase, due the static pressure at sea level (where the BOP is located). Hydrocarbons in liquid or dense phase are much less compressible than hydrocarbons in gas phase. A typical natural gas will go into dense phase if the pressure is above 153.5 bara (Cricondenbar) and temperature between −29° C. (Critical temperature) and +99° C. (Cricondentherm). As the gas (in liquid or dense phase) travels up the marine riser, the static pressure is reduced, and the gas goes from liquid/dense phase to gas/vapour phase and expands several hundred times.
When the gas is expanding in the riser it may fill the entire annulus, pushing the static column of mud back to the rig, even if the BOP is closed. As the static mud column is reduced and the gas travels up the riser, the mud will come back at an accelerating and increasing flow rate. When the diverter system is activated, this mud and gas will be diverted safely overboard.
On many rigs, a so-called “mud/gas separator” (MGS) has been utilized in the diverter system in an attempt to separate the mud from the gas and return the mud to the system, thus avoiding mud discharge to the sea. Publication “API RP 64, RECOMMENDED PRACTICE FOR DIVERTER SYSTEMS EQUIPMENT AND OPERATIONS”, issued by the American Institute of Technology (API) states in section 7.2.4, entitled “Inadvertent Gas Entry into the Riser”, that:                “Shallow gas flows are not the only application for a diverter system when using a marine riser. Gas may inadvertently enter the riser while drilling at any depth when the BOP is shut-in on a kick. Gas may also enter the riser if the rams leak after the BOP is closed. Gas in the riser may be safely removed by diverting the flow overboard. In some designs, a mud/gas separator is utilized in the diverter system to separate the gas from the mud and return the mud to the system. Again, the design should not allow the diverter to completely shut-in the well.”        
The way this is solved in the prior art is that the diverter element, the return flow line and the diverter lines have been closed at the same time, forcing the fluid returning from the riser to go up to the MGS located at a higher level. This is illustrated in FIG. 1, which is a disclosure on page 114 in the BP public report entitled “Deepwater Horizon Accident Investigation Report” (published Sep. 8, 2010).
The dangerous parts of this design is that the flow rate of mud returning from the riser is much higher than the design capacity of the MGS, resulting in filling of MGS and vent line. On most of the rigs with this system it is up to the driller (operating procedures) to open the diverter overboard valve if he believes that the returns flow exceeds the capacity of the MGS.
In some rigs, an extra high level trip in the MGS and/or high pressure trip in the diverter housing has been installed to automatically open the diverter overboard line on high level in MGS or high pressure in the diverter housing.
In either one of these designs, the dangerous part is that the available time in which to take the appropriate action, i.e. before the vent line of the MGS is completely filled, is very limited. At the time when high level in the MGS or high pressure in the diverter have been reached, the mud returning from the riser is in a highly accelerating mode and the time available for opening the diverter valve is very limited.
A slug of heavy mud accelerating up the MGS vent line followed by a two phase flow and finally a large gas release will create an increased pressure in the diverter housing and possible leakage in the slip joint resulting in gas being release under the rig at the slip joint connection. A worst case scenario of such an event is the Deepwater Horizon disaster.
The present inventor has devised and embodied the invention to overcome the shortcomings of the prior art and to obtain further advantages.